Blood coagulation prevents hemorrhage and also contributes to thrombotic diseases including myocardial infarction, stroke, and venous thromboembolism. A great deal has been learned about blood coagulation on the scale of a test tube, an artery, and a vein. However, the regulation of blood coagulation at the single cell level has been largely unexplored. Under ordinary conditions endothelial cells that line all arteries and veins have anticoagulant properties. However, following injury or stress, endothelial cells develop some procoagulant properties. Our preliminary data demonstrates that stressed or stimulated endothelial cells support assembly of the prothrombinase complex in focal areas on the cell while maintaining anticoagulant proteins on other parts of the cell. Localization of procoagulant activity is dependent upon regulated phosphatidylserine exposure and convex curvature of membrane regions. Thus our data suggest that stressed, viable endothelial cells can support procoagulant enzyme activity at the subcellular level while maintaining anticoagulant properties. The objectives of this proposal are to identify the locations of the three major procoagulant enzyme complexes on stressed endothelial cells in relation to details of cell structure and in relation to anticoagulant membrane proteins. We will explore the functional consequences of the localized enzyme complexes, particularly with regard to generation of small quantities of fibrin that bind to stressed endothelial cells or to the matrix adjacent to the cells. Finally, we will explore the contribution of a recently identified membrane protein named TMEM16F to the exposure of phosphatidylserine that occurs on stressed cells. The results of our studies are relevant to fundamental understanding of the blood coagulation mechanism and its participation in heart attacks and strokes. It is also related to the role of the blood coagulation mechanism in preventing or limiting bacterial infections. Finally, it may also help to explain the relationship of blood coagulation to inflammatory diseases.